The 'sendDigits' attribute tells Twilio to play DTMF tones when
the call is answered. This is useful when dialing a phone number and an
extension. Twilio will dial the number, and when the automated system picks up,
send the DTMF tones to connect to the extension.

The 'url' attribute allows you to specify a url for a TwiML document that will
run on the called party's end, after she answers, but before the parties are
connected. You can use this TwiML to privately play or say information to the
called party, or provide a chance to decline the phone call using <Gather>
and <Hangup>. The current caller will continue to hear ringing while the
TwiML document executes on the other end. TwiML documents executed in this
manner are not allowed to contain the <Dial> verb.

In this case, we want to dial the 1928 extension at 415-123-4567. We use
a <Number> noun to describe the phone number and give it the attribute
sendDigits. We want to wait before sending the extension, so we add a few
leading 'w' characters. Each 'w' character tells Twilio to wait 0.5 seconds
instead of playing a digit. This lets you adjust the timing of when the digits
begin playing to suit the phone system you are dialing.

In this case we use several <Number> tags to dial three phone numbers at the same time.
The first of these calls to answer will be connected to the current caller, while the rest of the
connection attempts are canceled.

You can specify up to ten numbers within a <Dial> verb to dial
simultaneously

Simultaneous dialing is useful when you have several phones (or several people)
that you want to ring when you receive an incoming call. Keep in mind that the first
call that connects will cancel all the other attempts. If you dial an office phone
system or a cellphone in airplane mode, it may pick up after a single ring,
preventing the other phone numbers from ringing long enough for a human to ever answer.

Hence you should take care to use simultaneous dialing in situations where you know the behavior of
the called parties.